Organosilicon compositions in which a platinum group metal-containing catalyst is inhibited in its cure-promoting activity at room temperature by the presence of a catalyst inhibitor are well known in the organosilicon art. For example, U.S. Pat. No. 4,256,870 issued to Eckberg teaches a method of producing a coating composition that has an improved bath life by mixing in any order a polysiloxane base polymer, a methylhydrogen crosslinking agent, a platinum catalyst, and diallylmaleate is added as an inhibitor to effectively retard the hydrosilation addition cure reaction of the composition at ambient temperature, but which does not retard the cure at elevated temperature.
U.S. Pat. No. 4,476,166 issued to Eckberg teaches a two-part inhibitor system that produces a solventless silicone release coating with improved bath life and cure time by mixing in any order an olefinorganopolysiloxane, an organohydrogenpolysiloxane as a crosslinking agent, a platinum catalyst, and a blend of diallylmaleate and vinyl acetate as an inhibitor.
U.S. Pat. No. 4,587,137 issued to Eckberg provides novel dual cure silicone compositions comprising (a) a polydiorganosiloxane containing silicon-bonded vinyl radicals and silicon-bonded hydrogen atoms, (b) a free radical photoinitiator such as t-butyl peroxybenzoate or mixtures of t-butyl peroxybenzoate and benzophenone, (c) a precious metal or precious metal containing hydrosilation catalyst, (d) optionally a organohydrogen polysiloxane, (e) optionally an olefin containing polyorganosiloxane, and (f) optionally an organic ester of maleic acid as an inhibitor to selectively retard the thermal addition cure reaction. These compositions were cured by exposure to UV light with the option of subsequently applying a thermal post bake after irradiation.
U.S. Pat. No. 4,262,107 issued to Eckberg provides a one-part or two-part inhibitor system which produces a paper release coating composition with an improved bath life and cure time by mixing in any order a silanol polymer, a methylhydrogen crosslinking agent, a rhodium catalyst, and a low molecular weight silanol endstopped diorganopolysiloxane alone or in combination with a diallylmaleate as an inhibitor.
U.S. Pat. No. 4,774,111 issued to Lo discloses a curable organosilicon composition comprising a component having silicon-bonded hydrogen atoms, a component having silicon-bonded olefinic hydrocarbon radicals reactive therewith, a platinum-containing catalyst and an effective amount of a diorgano fumarate cure control, i.e., catalyst inhibitor, component.
U.S. Pat. No. 4,783,552 issued to Lo et al. teaches that organosilicon compositions which cure by way of a metal-catalyzed reaction of silicon-bonded hydroxyl radicals and/or silicon-bonded olefinic hydrocarbon radicals with silicon-bonded hydrogen atoms are stabilized for hours at room temperature by the incorporation of a hydrocarbonoxyalkyl maleate.
U.S. Pat. No. 3,445,420 issued to Kookootsedes et al. provides a mixture of an olefin containing organosilicon polymer, an organosilicon compound containing silicon-bonded hydrogen atoms, a platinum catalyst and an acetylenic compound as inhibitor.
The maleates have been found to be particularly effective for increasing the room temperature bath life, i.e., work time, of solventless coating organosilicon compositions which cure by way of a platinum group metal-catalyzed reaction. However, the heating time and/or temperature needed to cure in these maleate-inhibited systems is sometimes excessive. When one attempts to decrease the cure time and/or temperature of silicone compositions to a commercially desirable interval by using less maleate and/or more catalyst in these inhibitor systems the bath life is frequently decreased to a commercially undesirable interval.
The fumarate inhibitor systems have been found to allow a cure of a solventless coating organosilicon compositions which cure by way of a platinum group metal-catalyzed reaction to take place at a suitable heating time and/or temperature. However, the bath life of such a composition, as measured by gel time at room temperature, is not as long as desired. When one attempts to increase the bath life of these compositions by increasing the amount of fumarate and or decreasing the amount catalyst in the fumarate inhibitor systems the cure and/or temperature increases.
In the coating arts, such as the paper coating art, the coating composition that is used to coat a substrate should not cure to the extent that its viscosity has increased substantially before it has been applied to the substrate; however, it should rapidly cure thereafter, preferably with only a moderate amount of added energy. Typically this means that the coating compositions preferably should not gel for as long as eight hours but should cure rapidly at moderately increased temperature to such an extent that the coated substrate can be further processed, if desired, without damaging the coating. In addition, the cure time of the composition at a given cure temperature desirably should remain substantially constant as the bath ages. Room temperature stability in a thin film is also important in this art, especially in 3-roll differential gravure coating where premature gelation of the material can clog the cells in the gravure roll thereby leading to a drop in silicone coatweight. In other coating methods such as 5-roll coating, insufficient thin film bath life can lead to the formation of residue on the rolls, thus leading to increased downtime for cleaning.
U.S. Pat. No. 5,036,117 issued to Chung et al. discloses that the bulk room temperature bath life of paper release formulations containing these types of inhibitors can be further extended without seriously effecting high temperature cure by adding substantially non-inhibitors such as benzyl alcohol. Chung et al. teaches that the preferred bath life extenders can be any organic or inorganic compound which is free of an inhibiting effect and has a Hansen partial solubility parameter for hydrogen bonding of 8.0, preferably 13-48, and is free of steric hindrance in the polar portion of the molecule (See column 11, lines 65-68 and column 12, lines 1-11).
U.S. Pat. No. 5,125,998 issued to Jones et al. provides a method for improving the bath life and/or cure time of curable compositions. The process comprises first mixing an inhibitor with a catalyst, then adding that mixture to an organosilicon compound, and then adding the resulting mixture to an organohydrogensilicon compound. The process can optionally comprise a bath life extender as taught by Chung et al.
While the art has proposed and provided some solutions for the problem, the quest for the ideal inhibitor package that allows for low temperature cure thermal solventless paper release products while at the same time maintaining reasonable bulk and thin film bath lives continues.